Pressure control unit and method facilitating single-phase heat transfer in a cooling system

ABSTRACT

A pressure control unit and method are provided for facilitating single-phase heat transfer within a liquid-based cooling system. The pressure control unit includes a pressure vessel containing system coolant, and a pressurizing mechanism associated with the pressure vessel. A coolant line couples system coolant in the pressure vessel in fluid communication with the coolant loop of the cooling system, and a regulator mechanism couples to the pressurizing mechanism to maintain pressure within the pressure vessel at or above a defined pressure threshold, thus maintaining pressure within the coolant loop above the pressure threshold. The defined pressure threshold is set to facilitate system coolant within the coolant loop remaining single-phase throughout an operational temperature range of the system coolant within the coolant loop. More particularly, the pressure threshold is set to ensure pressure of system coolant within the coolant loop remains above the coolant&#39;s saturation pressure at maximum operational temperature.

BACKGROUND

The present invention relates in general to heat transfer mechanisms,and more particularly, to cooling apparatuses and methods for removingheat generated by one or more electronic devices. Still moreparticularly, the present invention relates to fluidic coolingapparatuses and methods for cooling one or more electronic devices.

The industry trend has been to continuously increase the number ofelectronic devices within a computing system environment. Compactnessallows for selective fabrication of smaller and lighter devices that aremore attractive to the consumer. Compactness also allows circuits tooperate at higher frequencies and at higher speeds due to the shorterelectrical connection distances in such devices. Despite theseadvantages, providing many electronic devices in a small footprint cancreate device performance challenges. One of these challenges is thermalmanagement of the overall environment. Heat dissipation issues, ifunresolved, can result in electronic and mechanical failures that willaffect system performance, irrespective of the size of the environment.

In many computing environments, microprocessors continue to increase inperformance, with the active circuitry of the microprocessor chip beingdriven to an ever smaller footprint, leading to ever higher heat loadsand heat fluxes. Notwithstanding this, reliability constraints oftendictate that operating temperature of the devices not exceed a knownmaximum value.

The existing art has struggled with designing high-performance coolingsolutions that can efficiently remove this heat. Conventional coolingsolutions depend on conduction cooling through one or more thermalinterfaces to an air-cooled heat sink, possibly employing a spreader orvapor chamber. To increase the heat removal capability of air-cooledsystems, greater airflow is typically needed. Unfortunately, providinggreater airflow is not always possible. Many factors must be taken intoconsideration in providing ever greater airflow, among which areacoustic noise considerations, as well as power concerns.

As an alternative, liquid cooling methods have recently beenincorporated into certain designs. Various types of liquid coolantsprovide different cooling capabilities. For example, fluids such asrefrigerants or other dielectric liquids (e.g., fluorocarbon liquids)exhibit lower thermal conductivity and specific heat properties comparedto liquids such as water or other aqueous fluids. Dielectric liquidshave an advantage, however, in that they may be in direct physicalcontact with the electronic devices and their interconnects withoutadverse effects, such as corrosion or electrical short circuits.

BRIEF SUMMARY

To facilitate employing a dielectric fluid (such as a refrigerant) insingle-phase mode within a cooling system, it is desirable that theoperating pressure of the coolant be elevated above a saturationpressure to prevent vapor formation, and hence maintain the single-phaseoperating environment. This is accomplished herein, in one aspect, bymaintaining system coolant pressure within the coolant loop above thecoolant's saturation pressure for a given operational temperature rangeor, more particularly, for a maximum operational temperature of thesystem coolant within a coolant loop of a cooling system.

Briefly summarized, the present invention comprises in one aspect apressure control unit for facilitating single-phase heat transfer in acoolant loop of a cooling system. The pressure control unit includes: apressure vessel, having system coolant therein; a pressurizing mechanismassociated with the pressure vessel; a coolant line to couple the systemcoolant in the pressure vessel in fluid communication with the coolantloop of the cooling system; and a regulator mechanism coupled to thepressurizing mechanism to maintain pressure within the pressure vesselat or above a defined pressure threshold. By maintaining pressure withinthe pressure vessel at or above the defined pressure threshold, pressurewithin the coolant loop is maintained at or above the defined pressurethreshold through supply of system coolant from the pressure vessel intothe coolant loop when the coolant line couples the pressure vessel andcoolant loop in fluid communication. The defined pressure threshold isset to facilitate system coolant within the coolant loop remainingsingle-phase throughout an operational temperature range of the systemcoolant.

In another aspect, a cooling system is provided which includes: at leastone liquid-cooled cold plate configured to couple to at least oneelectronic device to be cooled; a coolant loop in fluid communicationwith the at least one liquid-cooled cold plate for facilitating flow ofsystem coolant through the at least one liquid-cooled cold plate; and apressure control unit coupled to the coolant loop for facilitatingsingle-phase system coolant heat transfer through the coolant loop. Thepressure control unit includes: a pressure vessel comprising systemcoolant; a pressurizing mechanism associated with the pressure vessel; acoolant line coupling the system coolant in the pressure vessel in fluidcommunication with the coolant loop; and a regulator mechanism coupledto the pressurizing mechanism to maintain pressure within the coolantloop at or above a defined pressure threshold through supply of systemcoolant from the pressure vessel into the coolant loop. The definedpressure threshold is set to facilitate system coolant within thecoolant loop remaining single-phase throughout an operationaltemperature range of the system coolant within the coolant loop.

In a further aspect, a method of facilitating single-phase systemcoolant heat transfer in a coolant loop of a cooling system is provided.The method includes: coupling a pressure vessel in fluid communicationwith the coolant loop of the cooling system, the pressure vesselcomprising system coolant; and regulating pressure within the pressurevessel to maintain the pressure at or above a defined pressurethreshold, and thereby maintain pressure within the coolant loop at orabove the defined pressure threshold through supply of system coolantfrom the pressure vessel into the coolant loop, wherein the definedpressure threshold is selected to facilitate system coolant within thecoolant loop of the cooling system remaining single-phase throughout anoperational temperature range of the system coolant within the coolantloop.

Further, additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic of one embodiment of a cooling system with apressure control unit, in accordance with an aspect of the presentinvention;

FIG. 2 is a schematic of an alternate embodiment of a cooling systemwith a pressure control unit, in accordance with an aspect of thepresent invention;

FIG. 3 is a schematic of a further embodiment of a cooling system with apressure control unit, in accordance with an aspect of the presentinvention;

FIG. 4A is a schematic of another embodiment of a cooling system with apressure control unit, in accordance with an aspect of the presentinvention;

FIG. 4B depicts one embodiment of logic to control pressure within thecoolant loop of the cooling system of FIG. 4A, in accordance with anaspect of the present invention;

FIG. 5 is a schematic of a further embodiment of pressure control unitcomponents for a cooling system, in accordance with an aspect of thepresent invention;

FIG. 6 is a schematic of another embodiment of pressure control unitcomponents for a cooling system, in accordance with an aspect of thepresent invention;

FIG. 7A is a schematic of one embodiment of a cooling system utilizingthe pressure control unit approach of FIG. 6, in accordance with anaspect of the present invention; and

FIG. 7B depicts one embodiment of logic to control pressure within thecoolant loop of the cooling system of FIG. 7A, in accordance with anaspect of the present invention.

DETAILED DESCRIPTION

As used herein, “electronic device” comprises any heat-generatingelectronic device of a computer system or other electronic systemrequiring cooling. In one example, the electronic device is or includesan integrated circuit chip, a semiconductor chip and/or any otherelectronic device requiring cooling, and may either be unpackaged orpackaged in an electronic module. As one example, the electronic devicemay comprise part of an electronic system disposed, for example, in anelectronics rack, such as a rack-mounted server system. A “liquid-to-airheat exchanger” means any heat exchange mechanism through which liquidcoolant can circulate; and includes, one or more discrete heat exchangedevices coupled either in series or in parallel. A heat exchange devicemay comprise, for example, one or more coolant flow paths, formed ofthermally conductive fluid conduit (such as copper, brass or othertubing) in thermal contact with a plurality of air-cooled fins (formedof a thermally conductive material, such as copper). Unless otherwisespecified, size, configuration and construction of the liquid-to-airheat exchanger can vary without departing from the scope of the presentinvention. A “liquid-to-liquid heat exchanger” may comprise, forexample, two or more coolant flow paths, formed of thermally conductivetubing (such as copper or other tubing) in thermal or mechanical contactwith each other. Size, configuration and construction of theliquid-to-liquid heat exchanger can also vary without departing from thescope of the invention disclosed herein. The term “liquid-cooled coldplate” refers to any thermally conductive structure having one or morechannels (or passageways) formed therein for flowing of liquid coolanttherethrough.

One example of system coolant employed in a cooling system such asdescribed herein is a dielectric liquid (such as a fluorocarbon or ahydrofluoroether (HFE) liquid) or a refrigerant liquid (such asR-245fa). As explained further below, a pressurized fluid, such as apressurized gas, is used (in one embodiment) in combination with excesssystem coolant to facilitate maintaining system coolant within thecoolant loop at or above a defined pressure threshold, for example, ator above saturation pressure of the system coolant for a given maximumoperational temperature, i.e., a given minimum saturation temperature ofthe system coolant within the coolant loop of the cooling system. Thispressurized fluid may comprise, for example, pressurized air,pressurized nitrogen or other pressurized gas. In one embodiment, thesystem coolant and the pressurized fluid are immiscible.

As noted, provided herein are a pressure control unit and method forfacilitating single-phase heat transfer within a coolant loop of acooling system. The cooling system described herein employs liquidcoolant-based cooling of one or more electronic devices utilizing one ormore liquid-cooled cold plates and a coolant loop in fluid communicationtherewith for facilitating flow of system coolant through the coldplates.

As noted, liquid-cooling of electronic device(s) using a dielectriccoolant has one significant advantage over water-based cooling. Should aleak occur, there is no physical harm (or electrical safety concern) dueto the dielectric liquid contacting the electronic device or othercircuitry. Unfortunately, thermal performance of dielectric coolants istypically poor in comparison with that of water, as set out in Table 1below.

TABLE 1 Attribute Unit Water HFE-7200 R245fa Density kg/m³ 998.2 1423.51352.2 Specific Heat J/kgK 4182 1214 1328.3 Thermal Conductivity W/mK0.600 0.068 0.092 Dynamic Viscosity mPa_s 1.003 0.673 0.432 RelativeFigure of Merit 14.3 1 2.2

Equation (1) below can be used to compare the relative laminar heattransfer capability of fluids at a given pumping power, defined as afigure of merit (FOM).

$\begin{matrix}{{FOM} = \frac{C_{p}k}{\mu}} & (1)\end{matrix}$

In Equation (1), C_(p) is specific heat, k is thermal conductivity and μis dynamic viscosity of the fluid. As can be seen, water has more thanan order of magnitude better performance than a representativedielectric coolant such as 3M Corporation's Novec™ Engineered FluidHFE-7200. Note, however, that the refrigerant R-245fa yields more thantwice the performance of that of HFE-7200 using the same figure ofmerit.

Water-based cooling systems have traditionally operated in single-phasemode. In single-phase mode, the coolant remains liquid throughout. Heatis transferred by sensible means, that is, there is no latent ortwo-phase heat transfer. A single-phase system is sometimes preferredover a two-phase system and is less expensive to implement. Further,two-phase systems are generally more complex to operate, particularlywith respect to flow distribution. However, if a fluid such as R-245farefrigerant is to be used in single-phase mode, then the systemoperating pressure needs to be elevated and maintained above atmosphericpressure by as much as 52 psi, as will be apparent to one skilled in theart using available saturation pressure versus saturation temperaturegraphs for R-245fa. Thus, disclosed herein is a pressure control unitand method for establishing and controlling pressure of system coolantwithin a coolant loop of an electronic cooling system to prevent vaporformation and hence maintain the single-phase operating environment.This is accomplished by maintaining the system pressure above saturationpressure for a given maximum operating temperature (i.e., a givenminimum saturation temperature) of the system coolant within the coolantloop of a given cooling system.

Reference is made below to the drawings, which are not drawn to scale tofacilitate understanding of the present invention, wherein the samereference numbers are used throughout different figures to designate thesame or similar components.

FIG. 1 illustrates one example of a cooling system, generally denoted100, in accordance with an aspect of the present invention. As shown,cooling system 100 comprises a liquid-based cooling system 101 and apressure control unit 102 coupled in fluid communication with a coolantloop 120 of liquid-based cooling system 101. Liquid-based cooling system101 includes, in this example, multiple electronic devices 110 to becooled and multiple cold plates 111 coupled thereto. Each cold plate 111is a liquid-cooled cold plate which is configured to couple to arespective electronic device to be cooled. To facilitate assembly andservicing, quick connect couplings 112 are provided (by way of example)in the individual source and return cold plate branches of coolant loop120. These quick connect couplings may be a source of pressure losswithin the coolant loop since a small amount of system coolant may belost with each connect or disconnect of a cold plate 111 from coolantloop 120. By way of example, quick connect couplings 112 may compriseany one of various types of commercially available couplings, such asthose available from Colder Products Company, of St. Paul, Minn.,U.S.A., or Parker Hannifin, of Cleveland, Ohio, U.S.A. System coolantwithin coolant loop 120 is pumped by a pump 121 through the one or morecold plates 111 of the liquid-based cooling system 101. After passingthrough the cold plates 111, excess heat is rejected from the systemcoolant in a heat exchanger 122, which may comprise an air-to-liquidheat exchanger or a liquid-to-liquid heat exchanger, before the coolantis returned to a reservoir 123 in fluid communication with coolant loop120.

System coolant within coolant loop 120 is maintained pressurized above adefined pressure threshold employing pressure control unit 102. Asshown, pressure control unit 102 includes a pressure vessel 130comprising system coolant 131 and a pressurized fluid 132. A coolantline 140 couples system coolant 131 within pressure vessel 130 in fluidcommunication with coolant loop 120 of liquid-based cooling system 101.In the embodiment illustrated, this coupling is at the suction side ofpump 121 (by way of example only). The suction side of a coolant pump istypically one location within a coolant loop where the pressure of thesystem coolant may be at its lowest value.

Pressure control unit 102 further includes a pressurizing mechanismassociated with pressure vessel 130. This pressurizing mechanismincludes, in the example illustrated in FIG. 1, pressurized fluid 132and a pressurized fluid source 160, such as a tank of compressed gas. Aregulator mechanism 150 is coupled to the pressurizing mechanism tomaintain pressure within pressure vessel 130 above the defined pressurethreshold, and thereby maintain pressure within the coolant loop at orabove the defined pressure threshold by supply of system coolant frompressure vessel 130 into coolant loop 120. As one example, regulatormechanism 150 is a mechanically operated pressure regulator value set toopen if pressure drops below the defined pressure threshold. As noted,the defined pressure threshold is set to facilitate system coolantwithin the coolant loop 120 of liquid-based cooling system 101 remainingsingle-phase throughout an operational temperature range of the systemcoolant within the coolant loop. In one specific example, the regulatormechanism is a pressure regulator valve, and the pressurized fluidsource 160 is a pressurized tank of nitrogen gas. The tank of nitrogengas is assumed to be significantly higher in pressure than the desiredsystem coolant operating pressure (i.e., the defined pressurethreshold), with the pressure regulator valve maintaining pressurewithin the pressure vessel at or above the desired pressure. In the caseof an R-245fa system coolant, that pressure might be 52 psig for amaximum operating temperature of 60° C.

Note that various modifications to the pressure control unit depicted inFIG. 1 are possible. For example, rather than employing a separatepressure vessel 130, reservoir 123 within the liquid-based coolingsystem could itself be the pressure vessel described herein. Further,the concepts disclosed herein are applicable to any combination ofparallel and/or series connected cold plates 111. Also, fluid connectionfrom pressure vessel 130 into coolant loop 120 can be to any part of thecoolant loop. This is illustrated in FIG. 2, wherein a cooling system200 is shown to comprise liquid-based cooling system 101, describedabove in connection with FIG. 1, and a pressure control unit 102′ suchas pressure control unit 102 described above in connection with FIG. 1.The difference between the cooling system embodiments of FIGS. 1 & 2 isthat coolant line 201 coupling pressure vessel 130 of pressure controlunit 102′ in FIG. 2 is to coolant loop 120 at a location between coldplates 111 and heat exchanger 122 of the liquid-based cooling system101. By placing the coolant line connection in this location, theresultant system ensures sufficient pressure so that system coolantpassing through liquid-cooled cold plates 111 remains single-phase.

FIG. 3 depicts a further embodiment of a cooling system 300, inaccordance with an aspect of the present invention. In this embodiment,cooling system 300 includes liquid-based cooling system 101, describedabove in connection with FIG. 1, and a pressure control unit 302 forfacilitating single-phase heat transfer within coolant loop 120 ofliquid-based cooling system 101. Pressure control unit 302 includes apressure vessel 330 comprising system coolant 331 and a pressurizedfluid 332. In this embodiment, system coolant 331 and pressurized fluid332 are separated by a flexible bladder 310, for example, a flexiblerubber bladder attached to the inner wall of pressure vessel 330. Acoolant line 340 couples system coolant 331 in pressure vessel 330 influid communication with coolant loop 120 of the cooling system.

By way of example, pressure control unit 302 includes a pressure reliefvalve 320 coupled to pressure tank 330 for ensuring that pressure withinthe pressure tank does not exceed an upper limit. The pressurizingmechanism includes (in this example) pressurized fluid 332 and acompressor 360. By way of further example, pressurized fluid 332 ispressurized air, and compressor 360 is activated and deactivated toprovide a source of pressurized air to facilitate controlling pressurewithin pressure vessel 330. A pressure switch 361 is provided forautomated activation/deactivation of compressor 360 to ensure asufficient level of compressed air at the inlet to pressure vessel 330.Regulator mechanism 350 may comprise a pressure regulator valve whichautomatically mechanically opens and closes to adjust pressure at theinlet to pressure tank 330. As one example, the pressure regulator valvemay comprise a spring-loaded valve stem which opens to let pressurizedgas into the pressure vessel whenever pressure drops below a defined setpoint, and closes once pressure within the vessel equals or exceeds thedefined set point. Note that in this embodiment, there is no requirementthat pressurized fluid 332 and system coolant 331 be immiscible fluidssince they are physically separated by flexible bladder 310. Abladder-type pressure vessel (or tank) is well known in the field ofwater distribution. In the embodiment of FIG. 3, a similar type vesselmay be employed as the pressure vessel, with the bladder separating thesystem coolant and pressurized fluid.

FIG. 4A illustrates a further embodiment of a cooling system 400, inaccordance with an aspect of the present invention. Cooling system 400includes a liquid-based cooling system 101 (such as described above inconnection with FIG. 1) and a pressure control unit 402. As illustrated,a coolant line 440 couples in fluid communication reserve system coolant432 within pressure vessel 430 to coolant loop 120 of liquid-basedcooling system 101. A pressurized fluid 431 is also shown withinpressure vessel 430 to facilitate maintaining system coolant within thepressure vessel at or above a defined pressure threshold. By maintainingsystem coolant within pressure vessel 430 at or above the definedpressure threshold, system coolant within coolant loop 120 is similarlymaintained at or above the defined pressure threshold. In thisembodiment, a pressure sensor 410 (e.g., pressure transducer) is coupledto a pressurized fluid inlet to pressure vessel 430 and to a controller420. Controller 420 is also connected to the regulator mechanism 450,which is coupled between (in this embodiment) a pressurized fluid source460 and pressure vessel 430. In the illustrated embodiment, regulatormechanism 450 is assumed to comprise an electrically actuated valve(EV). Controller 420 is provided with logic to automatically control andmaintain pressure within pressure vessel 430 at or above the definedpressure threshold (P0).

FIG. 4B illustrates one embodiment of logic to automatically controlpressure within pressure vessel 430 of pressure control unit 402 ofcooling system 400 illustrated in FIG. 4A. Referring to FIG. 4B,pressure P is read 470 and a determination is made whether pressure P isat or above the defined pressure threshold (P0) 472. If “no”, then theelectrically actuated valve (EV) of the pressure control unit is openedby a set amount “x” 474, and the logic waits a defined time T 476 beforeagain reading pressure P 470. If pressure P is above the definedpressure threshold (P0), then the controller logic determines whetherthe electrically actuated valve (EV) is closed 478, and if “yes”, waitstime T 476 before again reading pressure P 470. If the electricallyactuated valve is not closed, then the valve is closed 480, and controllogic waits time T 476 before again reading pressure P 470.

FIGS. 5 & 6 depict alternate embodiments of a pressure control unitapproach, in accordance with an aspect of the present invention.

In FIG. 5, a pressure control unit approach is illustrated comprising apressure vessel 500 which includes system coolant 510 and has a coolantline 520 connected thereto which is sized and configured to couple thesystem coolant within the pressure vessel to a coolant loop of a coolingsystem (not shown). In this embodiment, a piston 530, positioned withinpressure vessel 500, is driven by a stepper motor 540. A controller (notshown) automatically controls stepwise extension or retraction of piston530 based on a pressure associated with pressure vessel 500 or thepressure of system coolant within the coolant loop of the cooling systemto which the pressure control unit is coupled. Note that this embodimentassumes that there is elasticity in the coolant loop of the coolingsystem, for example, the coolant loop comprises rubber or flexibleplastic hoses.

In FIG. 6, a piston 650 and stepper motor 660 arrangement areillustrated, with a pressurized fluid 640 (such as dry air or nitrogengas) shown disposed between a flexible bladder 630 and system coolant610 within pressure vessel 600. A coolant line 620 is sized andconfigured to couple system coolant 610 within pressure vessel 600 witha coolant loop of a cooling system (not shown). As noted above, thiscoupling to the coolant loop can be to any location in the coolant loop.

FIG. 7A depicts one embodiment of a cooling system 700 employing thecomponents of the pressure control unit approach depicted in FIG. 6.Cooling system 700 includes liquid-based cooling system 101 (describedabove in connection with FIG. 1) and a pressure control unit 702. Asshown, pressure control unit 702 includes a pressure vessel 600 whichcomprises system coolant 610 coupled via a coolant line 620 in fluidcommunication with coolant loop 120 of liquid-based cooling system 101.In this embodiment, coolant line 620 couples to coolant loop 120 betweenpump 121 and reservoir 123, by way of example only. In the depictedpressure control unit, pressure vessel 600 is shown to include aflexible bladder 630 separating system coolant 610 from pressurizedfluid 640, such as air or nitrogen gas. Piston 650 is reciprocated via astepper motor 660 to either increase or decrease system coolant pressurewithin pressure vessel 600. A controller 710 is coupled to stepper motor660 for automatically controlling movement of piston 650 within pressurevessel 600, and a pressure sensor 105 is provided between reservoir 123and pump 121 (in this example) to sense pressure of system coolantwithin coolant loop 120 of liquid-based cooling system 101. Acommunication line 720 couples controller 710 and pressure sensor 105 toallow logic within controller 710 to ascertain system coolant pressurewithin the coolant loop.

FIG. 7B depicts one embodiment of logic for facilitating single-phaseheat transfer in the coolant loop of the cooling system of FIG. 7Aemploying the pressure control unit depicted therein. The logic, whichis implemented by controller 710, reads pressure P of system coolantwithin the coolant loop 750, and determines whether pressure P is belowa first, low pressure threshold PL 752. If “yes”, then the workingvolume within the pressure vessel is decreased by a set amount “x” 754,with the result of increasing pressure on the system coolant, afterwhich the logic waits a time T 756 before again determining the pressureP within the coolant loop 750. If the read pressure P is greater orequal to the low pressure threshold PL, then the logic determineswhether pressure P is above a high pressure threshold PH 758. If “yes”,then the working volume within the pressure vessel is increased by theset amount “x” 760, with the result of reducing pressure on the systemcoolant within the pressure vessel, and thus, reducing pressure onsystem coolant within the coolant loop of the cooling system, forexample, by drawing system coolant from the coolant loop into thepressure vessel.

Note that the above-described cooling systems, pressure control unitsand methods are provided by way of example only. Numerous variations tothe embodiments disclosed herein are possible without departing from thescope of the present invention. For example, the control logic to adjustpressure within the pressure vessel, and hence, within the coolant loop,could sense pressure at an inlet or outlet of the pressure vessel,within the pressure vessel, or at any location within the coolant loopof the cooling system. Further, the system coolant to be maintainedsingle-phase within the coolant loop may comprise any type of liquidcoolant appropriate for an electronics cooling system.

Further details and variations on liquid-based cooling apparatuses andmethods for cooling electronics systems and/or electronics racks aredisclosed in co-filed U.S. patent application Ser. No. ______, entitled“Control of System Coolant to Facilitate Two-Phase Heat Transfer in aMulti-Evaporator Cooling System”, (Attorney Docket No. POU920090068US1),and co-filed U.S. patent application Ser. No. ______, entitled “Systemand Method for Facilitating Parallel Cooling of Liquid-CooledElectronics Racks”, (Attorney Docket No. POU920090085US1), and co-filedU.S. patent application Ser. No. ______, entitled “Cooling System andMethod Minimizing Power Consumption in Cooling Liquid-Cooled ElectronicsRacks”, (Attorney Docket No. POU920090087US1), and co-filed U.S. patentapplication Ser. No. ______, entitled “Apparatus and Method forAdjusting Coolant Flow Resistance Through Liquid-Cooled ElectronicsRack(s)”, (Attorney Docket No. POU920090078US1), the entirety of each ofwhich is hereby incorporated herein by reference.

As will be appreciated by one skilled in the art, aspects of thecontroller described above may be embodied as a system, method orcomputer program product. Accordingly, aspects of the controller maytake the form of an entirely hardware embodiment, an entirely softwareembodiment (including firmware, resident software, micro-code, etc.) oran embodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit”, “module” or “system”.Furthermore, aspects of the controller may take the form of a computerprogram product embodied in one or more computer readable medium(s)having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readablestorage medium. A computer readable storage medium may be, for example,but not limited to, an electronic, magnetic, optical, or semiconductorsystem, apparatus, or device, or any suitable combination of theforegoing. More specific examples (a non-exhaustive list) of thecomputer readable storage medium include the following: an electricalconnection having one or more wires, a portable computer diskette, ahard disk, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), anoptical fiber, a portable compact disc read-only memory (CD-ROM), anoptical storage device, a magnetic storage device, or any suitablecombination of the foregoing. In the context of this document, acomputer readable storage medium may be any tangible medium that cancontain or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

A computer-readable signal medium may include a propagated data signalwith computer-readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electromagnetic, optical, or any suitable combination thereof Acomputer-readable signal medium may be any computer-readable medium thatis not a computer-readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus or device.

Program code embodied on a computer readable medium may be transmittedusing an appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programminglanguage, such as Java, Smalltalk, C++ or the like, and conventionalprocedural programming languages, such as the “C” programming languageor similar programming languages.

Aspects of the present invention are described above with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowcharts or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblocks may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

Although embodiments have been depicted and described in detail herein,it will be apparent to those skilled in the relevant art that variousmodifications, additions, substitutions and the like can be made withoutdeparting from the spirit of the invention and these are thereforeconsidered to be within the scope of the invention as defined in thefollowing claims.

1. A pressure control unit for facilitating single-phase heat transferin a coolant loop of a cooling system, the pressure control unitcomprising: a pressure vessel comprising system coolant; a pressurizingmechanism associated with the pressure vessel; a coolant line to couplethe system coolant in the pressure vessel in fluid communication withthe coolant loop of the cooling system; and a regulator mechanismcoupled to the pressuring mechanism to maintain pressure within thepressure vessel at or above a defined pressure threshold, and therebymaintain pressure within the coolant loop at or above the definedpressure threshold through supply of system coolant from the pressurevessel into the coolant loop when the coolant line couples the pressurevessel and coolant loop in fluid communication, and wherein the definedpressure threshold is set to facilitate system coolant within thecoolant loop of the cooling system remaining single phase throughout anoperational temperature range of system coolant within the coolant loop.2. The pressure control unit of claim 1, wherein the operationaltemperature range comprises a maximum operational temperature of thesystem coolant, and the defined pressure threshold is set higher thansaturation pressure of the system coolant at the maximum operationaltemperature.
 3. The pressure control unit of claim 1, wherein thepressurizing mechanism comprises a pressurized fluid source coupled tothe pressure vessel via the regulator mechanism, wherein the regulatormechanism automatically adjusts pressure within the pressure vessel bysupplying pressurized fluid from the pressurized fluid source to thepressure vessel when pressure within at least one of the pressure vesselor the coolant loop drops below the defined pressure threshold.
 4. Thepressure control unit of claim 3, wherein the pressurized fluid sourcecomprises one of a tank of compressed gas or an air compressor.
 5. Thepressure control unit of claim 1, wherein the pressurizing mechanismcomprises a pressurized fluid, and the system coolant and thepressurized fluid are immiscible fluids.
 6. The pressure control unit ofclaim 1, wherein the pressurizing mechanism comprises a pressurizedfluid within the pressure vessel, and the pressure control unit furthercomprises a bladder disposed within the pressure vessel separating thesystem coolant and the pressurized fluid.
 7. The pressure control unitof claim 1, wherein the pressurizing mechanism comprises a pressurizedfluid within the pressure vessel, and the pressure control unit furthercomprises: a pressure sensor for determining pressure of system coolantwithin one of the pressure vessel or the coolant loop of the coolingsystem; and a controller coupled to the pressure sensor and to theregulator mechanism, wherein the regulator mechanism comprises aproportional valve and the controller controls at least one of openingor closing of the proportional valve to maintain pressure of thepressurized fluid within the pressure vessel at or above the definedpressure threshold, and thereby maintain pressure of the system coolantwithin the pressure vessel at or above the defined pressure threshold.8. The pressure control unit of claim 1, wherein the pressurizingmechanism comprises a piston disposed within the pressure vessel, andwherein the regulator mechanism comprises a stepper motor coupled to thepiston for adjusting position of the piston within the pressure vesseland thus pressure of system coolant within the pressure vessel, andwherein the pressure control unit further comprises: a pressure sensorfor sensing pressure of system coolant within one of the pressure vesselor the coolant loop of the cooling system; and a controller coupled tothe pressure sensor and to the stepper motor, wherein the controllercontrols positioning of the piston within the pressure vessel via thestepper motor to maintain pressure of system coolant within the pressurevessel at or above the defined pressure threshold.
 9. The pressurecontrol unit of claim 8, wherein the pressurizing mechanism furthercomprises a pressurized fluid disposed within the pressure vessel, andwherein the pressure control unit further comprises: a bladder disposedwithin the pressure vessel separating the system coolant and thepressurized fluid disposed therein, wherein the pressurized fluid isdisposed between the bladder and the piston, and the pressurized fluidcomprises a pressurized gas.
 10. A cooling system comprising: at leastone liquid-cooled cold plate configured to couple to at least oneelectronic device to be cooled; a coolant loop in fluid communicationwith the at least one liquid-cooled cold plate for facilitating flow ofsystem coolant through the at least one liquid-cooled cold plate; and apressure control unit coupled to the coolant loop for facilitatingsingle-phase system coolant heat transfer through the coolant loop, thepressure control unit comprising: a pressure vessel comprising systemcoolant; a pressurizing mechanism associated with the pressure vessel; acoolant line coupling the system coolant in the pressure vessel in fluidcommunication with the coolant loop; and a regulator mechanism coupledto the pressurizing mechanism to maintain pressure within the coolantloop at or above a defined pressure threshold through supply of systemcoolant from the pressure vessel into the coolant loop, and wherein thedefined pressure threshold is set to facilitate system coolant withinthe coolant loop remaining single phase throughout an operationaltemperature range of the system coolant within the coolant loop of thecooling system.
 11. The cooling system of claim 10, wherein theoperational temperature range comprises a maximum operationaltemperature of the system coolant, and the defined pressure threshold isset higher than saturation pressure of the system coolant at the maximumoperational temperature
 12. The cooling system of claim 10, wherein thepressurizing mechanism comprises a pressurized fluid source coupled tothe pressure vessel via the regulator mechanism, wherein the regulatormechanism automatically adjusts pressure within the pressure vessel bysupplying pressurized fluid from the pressurized fluid source to thepressure vessel when pressure within at least one of the pressure vesselor the coolant loop drops below the defined pressure threshold, andwherein the system coolant and the pressurized fluid are immisciblefluids.
 13. The cooling system of claim 10, wherein the pressurizingmechanism comprises a pressurized fluid within the pressure vessel, andthe pressure control unit further comprises a bladder disposed withinthe pressure vessel separating the system coolant and the pressurizedfluid.
 14. The cooling system of claim 10, wherein the pressurizingmechanism comprises a pressurized fluid within the pressure vessel, andthe pressure control unit further comprises: a pressure sensor fordetermining pressure of system coolant within one of the pressure vesselor the coolant loop of the cooling system; and a controller coupled tothe pressure sensor and to the regulator mechanism, wherein theregulator mechanism comprises a proportional valve and the controllercontrols at least one of opening or closing of the proportional valve tomaintain pressure of the pressurized fluid within the pressure vessel ator above the defined pressure threshold, and thereby maintain pressureof the system coolant within the pressure vessel at or above the definedpressure threshold.
 15. The cooling system of claim 10, wherein thepressurizing mechanism comprises a piston disposed within the pressurevessel, and wherein the regulator mechanism comprises a stepper motorcoupled to the piston for adjusting position of the piston within thepressure vessel and thus pressure of system coolant within the pressurevessel, and wherein the pressure control unit further comprises: apressure sensor for sensing pressure of system coolant within one of thepressure vessel or the coolant loop of the cooling system; and acontroller coupled to the pressure sensor and to the stepper motor,wherein the controller controls positioning of the piston within thepressure vessel via the stepper motor to maintain pressure of systemcoolant within the pressure vessel at or above the defined pressurethreshold.
 16. The cooling system of claim 15, wherein the pressurizingmechanism further comprises a pressurized fluid disposed within thepressure vessel, and wherein the pressure control unit furthercomprises: a bladder disposed within the pressure vessel separating thesystem coolant and the pressurized fluid disposed therein, wherein thepressurized fluid is disposed between the bladder and the piston, andthe pressurized fluid comprises a pressurized gas.
 17. A method offacilitating single-phase system coolant heat transfer in a coolant loopof a cooling system, the method comprising: coupling a pressure vesselin fluid communication with the coolant loop of the cooling system, thepressure vessel comprising system coolant; and regulating pressurewithin the pressure vessel to maintain the pressure at or above adefined pressure threshold, and thereby maintain pressure within thecoolant loop at or above the defined pressure threshold through supplyof system coolant from the pressure vessel into the coolant loop,wherein the defined pressure threshold is selected to facilitate systemcoolant within the coolant loop of the cooling system remainingsingle-phase throughout an operational temperature range of the systemcoolant within the coolant loop.
 18. The method of claim 17, furthercomprising monitoring pressure of system coolant within at least one ofthe pressure vessel or the coolant loop of the cooling system, andwherein the regulating is responsive to the pressure monitoring.
 19. Themethod of claim 17, further comprising providing a pressurized fluidwithin the pressure vessel, and the regulating comprises regulating anamount of pressurized fluid within the pressure vessel, therebyregulating pressure within the pressure vessel, wherein the pressurizedfluid and the system coolant are immiscible fluids.
 20. The method ofclaim 17, wherein the operational temperature range of the systemcoolant within the coolant loop comprises a maximum operationaltemperature, and the method further comprises setting the definedpressure threshold higher than saturation pressure of the system coolantat the maximum operational temperature.